1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member. More particularly, it relates to an electrophotographic photosensitive member comprising an intermediate layer provided between a conductive support and a photosensitive layer.
2. Related Background Art
In general, in electrophotographic photosensitive members of a Carson type, the stabilities at dark portion potential and light portion potential are important in order to form a good image having a constant image density and also free of defects in the course of the repetition of charging and exposure.
For this reason, it has been hitherto proposed that an intermediate layer having the functions of, e.g., improving the injection of charge from a support to a photosensitive layer, improving the adhesion between a support and a photosensitive layer, and improving coating performance for a photosensitive layer is provided between a support and a photosensitive layer.
A number of proposals have been also made on the so-called function-separated photosensitive member, comprising a photosensitive layer having a laminated structure formed of a charge generation layer and a charge transport layer. In this case, the charge generation layer is usually provided as a very thin layer with a thickness of, for example, about 0.5 .mu.m, and its film thickness non-uniformity is closely related with the sensitivity non-uniformity of a photosensitive member. As the cause of the film thickness non-uniformity may be named defects, scratches or stains on the surface of a photosensitive member. From this viewpoint also, the intermediate layer is considered to be of very high necessity.
As materials to form the layer provided between a photosensitive member and a support, it is conventionally known to use polyamides (Japanese Patent Laid-open No. 46-47344 and No. 52-25638), polyesters (Japanese Patent Laid-open No. 54-20836 and No. 54-26738), casein (Japanese Patent Laid-open No. 55-103556), polypeptides (Japanese Patent Laid-open No. 53-48523), polyvinyl alcohols (Japanese Patent Laid-open No. 52-100240), polyvinyl pyrrolidone (Japanese Patent Laid-open No. 48-30936), a vinyl acetate/ethylene copolymer (Japanese Patent Laid-open No. 48-26141), a maleic anhydride ester polymer (Japanese Patent Laid-open No. 52-10138), polyvinyl butyral (Japanese Patent Laid-open No. 57-90639 and No. 58-106549), quaternary ammonium salt-containing polymers (Japanese Patent Laid-open No. 51-126149 and No. 56-60448), ethyl cellulose (Japanese Patent Laid-open No. 55-143564), etc.
In photosensitive members that use the above materials as the intermediate layer, however, the potential tends to change under the environmental influence of temperature and humidity, and it has been difficult to obtain potential characteristics and images that can be always stable to all environmental conditions of from the low temperature and low humidity to the high temperature and high humidity.
For example, when a photosensitive member is repeatedly used under conditions of low temperature and low humidity that increase the resistance of the intermediate layer, electric charge remains in the intermediate layer and hence the light portion potential and residual potential increase to cause fog on a copied image. When such a photosensitive member is used in a printer of an electrophotographic system in which reversal development is carried out, there have been the problems that the resulting image has a low density and no copies with constant image quality are obtainable.
Under conditions of high temperature and high humidity, the function as a barrier is lowered because of the intermediate layer having turned to have a low resistance, resulting in a lowering of the dark portion potential because of an increase in the injection of carriers from the support side. Thus, under conditions of the high temperature and high humidity, there have been the problems that the resulting copied image has a low density, and faulty black dots or fogged images tend to be formed when such a photosensitive member is used in the printer of an electrophotographic system in which reversal development is carried out.
Particularly in a lamination type electrophotographic photosensitive member comprising a photosensitive layer formed of a charge generation layer and a charge transport layer which are successively laminated, a charge generation layer containing a charge-generating material in a high concentration is positioned on an intermediate layer in contact therewith. Hence, a lowering of potential tends to occur because of an increase in the injection of carriers from the support side, so that even a slight lowering of the barrier function of the intermediate layer tends to generate fog in the printer of a reverse development type.
As a countermeasure to this problem, proposals have been made on an intermediate layer with a system comprising a conductive powder dispersed in a binder resin having a relatively high resistivity, as exemplified by a conductive powder dispersed in a polyester polyurethane (Japanese Patent Laid-open No. 61-163346), titanium (IV) oxide or tin (II) oxide dispersed in an acrylic polyurethane (Japanese Patent Laid-open No. 62-280863), and carbon black dispersed in an acrylic polyol or an isocyanate (Japanese Patent Laid-open No. 62-115467). Such systems can reduce characteristics variations due to temperature and humidity, but are comprised of a resin part having a high resistivity and a powder part having a very high conductivity which are present together. Hence, charge tends to be non-uniformly injected from the support side into a photosensitive layer. This may result in a lowering of potential in the case of the printer of a reverse development system, bringing about the problem that faulty black dots or fogged images tend to be formed.
When a photosensitive member comprised of such an intermediate layer is repeatedly used, charge is accumulated at the resin part having a high resistivity, with an increase in the speed of electrophotographic processing, to bring about another problem that the residual potential may increase.